Discharger and discharger control method

ABSTRACT

Upon detecting an external signal which instructs to stop discharge, an input voltage equal to or less than a set value for the prevention of overdischarge, or an output voltage equal to or more than a set value for the prevention of output of an overvoltage, a control unit ( 12 ) stops discharge to a load ( 40 ) by opening a switching element ( 4   b ) of a step-down unit ( 11   b ). Upon detecting that an external signal is reset or an input voltage equal to or more than a set value larger than the set value for the prevention of overdischarge, the control unit ( 12 ) resumes discharge to the load ( 40 ) by setting the switching element ( 4   b ) in a switching operation state or short-circuiting it.

The present patent application is a Utility claiming the benefit ofApplication No. PCT/JP2007/060691, filed May, 25, 2007.

TECHNICAL FIELD

The present invention relates to a discharger and a discharge controlmethod and, more particularly, to a discharger and discharge controlmethod which control an output from a DC power supply which is suppliedto a load when a battery system constituted by a plurality of assembledbatteries is used as a DC power supply system.

BACKGROUND ART

Nickel metal hydride batteries used for DC power supplies arecharacterized in that they have high energy densities and long batterylives and are environment-friendly as compared with lead-acid batteries.In addition, the nickel metal hydride batteries are compact andlightweight and have good portability, and hence have recently andrapidly become widespread as on-vehicle batteries and power supplies forcountermeasures against disaster.

In order to cope with a recent abrupt increase in power demand forcommunication equipment, it is necessary to form a power supply system(battery system) having a large capacity, e.g., an output of 30 kWh, byconnecting battery sets such as nickel metal hydride batteries inparallel with each other so as to increase the capacity of a DC powersupply. In general, when nickel metal hydride batteries are to be usedas a power supply system, in order to increase the capacity and servicelife of the power supply system, a large-capacity nickel metal hydridebattery system is formed by connecting k single nickel metal hydridebatteries (with an average voltage of 1.2 V and a current capacity of 95Ah) called single cells in series with each other to form one unit (tobe referred to as a module hereinafter), connecting m modules in serieswith each other to form an assembled battery, and connecting n assembledbatteries in parallel with each other.

Various proposals have been made for mechanisms for managing the supplycapacity of power for a load in a large-capacity battery system. Forexample, Japanese Patent Laid-Open Nos. 2004-119112, 2004-120856, and2004-120857 disclose management methods in power supply systems eachincluding a plurality of assembled batteries connected in parallel witheach other, a charge control means, and a discharge control means.

Japanese Patent Laid-Open No. 2004-119112 discloses a technique ofinternally storing the manufacturing date of an assembled battery,calculating an available period during which predetermined power can besupplied, on the basis of the stored manufacturing date of the assembledbattery, and displaying an assembled battery replacement date, in orderto facilitate estimation of the service life of the assembled battery atthe time of maintenance inspection.

Japanese Patent Laid-Open No. 2004-120856 discloses a technique ofproviding a battery monitoring means for executing a discharge capacitytest on a given assembled battery as a deterioration determinationtarget upon fully charging assembled batteries other than thedeterioration determination target as well as the target assembledbattery in order to allow power supply to the load side even if a powerfailure or the like occurs during the execution of the dischargecapacity test for deterioration determination of the assembled battery.

In addition, Japanese Patent Laid-Open No. 2004-120857 discloses atechnique of providing, for the purpose of reducing a power cost byleveling power demands, a battery monitoring means for monitoringwhether the residual capacity of an assembled battery has become equalto or less than a charge start threshold, and if the residual capacityhas become equal to or less than the charge start threshold, startingauxiliary charging of the assembled battery at a predetermined time lateat night when power is used relatively less.

DISCLOSURE OF INVENTION Problem to be Solved by the Invention

As described above, when a battery system with an output of 30 kWh is tobe implemented by using, for example, nickel metal hydride batteries, 10nickel metal hydride battery cells with a rated voltage of 1.2 V (anaverage voltage of 1.2 V and a current capacity of 95 Ah) are connectedin series to form one module, and four modules are connected in seriesto form one system of assembled batteries (output of 5 kWh). Inaddition, as shown in FIG. 3, six systems of assembled batteries areconnected in parallel with each other. FIG. 3 is a view showing thearrangement of a battery system formed by using a plurality of assembledbatteries and a plurality of dischargers. FIG. 3 also shows chargers forcharging the assembled batteries.

That is, in the arrangement example shown in FIG. 3, six systems ofassembled batteries 30-1, 30-2, 30-3, 30-4, 30-5, and 30-6 are connectedin parallel with each other to form a battery system using a pluralityof assembled batteries. This system further includes dischargers 10 forstepping up/stepping down the battery voltages respectively output fromthe six systems of assembled batteries so as to make the voltages fallwithin the allowable voltage range of a load 40, chargers 20 forintermittently charging a plurality of assembled batteries 30 from arectifier 50, and a power supply control unit 60 which controls theoverall operation of the battery system including the dischargers 10 andthe chargers 20. In the case shown in FIG. 3, a discharger 10-1 and acharger 20-1 are provided for the assembled batteries 30-1 and 30-2, adischarger 10-2 and a charger 20-2 are provided for the assembledbatteries 30-3 and 30-4, and a discharger 10-3 and a charger 20-3 areprovided for the assembled batteries 30-5 and 30-6.

In the battery system in FIG. 3, the assembled batteries 30, i.e., 30-1,30-2, 30-3, 30-4, 30-5, and 30-6, are charged by outputs from therectifier 50 via the corresponding chargers 20, i.e., 20-1, 20-2, and20-3, and power is supplied to the load 40 via the respectivedischargers 10, i.e., 10-1, 10-2, and 10-3 in accordance with controloperation by the power supply control unit 60. In this case, theplurality of dischargers 10 are electrically connected on the outputside and connected to the load 40. The plurality of chargers 20 areelectrically connected on the input side and connected to the rectifier50. Additionally installing dischargers 10, chargers 20, and assembledbatteries 30 can extend the battery system. For example, connectingthree 30-kWh battery systems, each shown in FIG. 3, in parallel witheach other can implement a battery system in the 100 kWh class.

The dischargers 10-1, 10-2, and 10-3 perform step-down operation(step-down mode) by using DC-DC converters if the battery voltagesrespectively output from the corresponding assembled batteries 30-1,30-2, 30-3, 30-4, 30-5, and 30-6 exceed the allowable voltage range ofthe load 40. If the battery voltages fall within the allowable voltagerange of the load 40 and the operating voltage range, the dischargersbypass (bypass mode) the battery outputs without via the DC-DCconverters. If the battery voltages fall below the operating voltagerange of the load 40, the dischargers perform step-up operation (step-upmode) using the DC-DC converters.

That is, the dischargers 10-1, 10-2, and 10-3 mounted in the 30-kWhbattery system shown in FIG. 3 each include a step-up unit 11 a and astep-down unit 11 b, as shown in FIG. 4. FIG. 4 is a circuit diagramshowing an example of the circuit arrangement of a step-up/step-downDC-DC converter which is mounted in a discharger, and shows an exampleof the circuit arrangement of the step-up unit 11 a and step-down unit11 b which constitute the step-up/step-down DC-DC converter. Asindicated by a discharger 10A in FIG. 4, the step-up unit 11 a and thestep-down unit 11 b each include circuit constituent elements. Thestep-up unit 11 a includes a reactor 1 a, a diode 2 a, a capacitor 3 a,and a switching element 4 a. The step-down unit 11 b includes a reactor1 b, a diode 2 b, a capacitor 3 b, and a switching element 4 b.

The step-up unit 11 a controls the switching element 4 a to performstep-up operation if the voltage applied from the assembled battery 30to the load 40 falls below the operating voltage range of the load 40.The step-down unit 11 b controls the switching element 4 b to performstep-down operation if the voltage applied from the assembled battery 30to the load 40 exceeds the allowable voltage range of the load 40.

In the arrangement of the discharger 10A like that shown in FIG. 4, evenafter discharge operation continues and the voltage of the assembledbattery 30 decreases to reach an overdischarge voltage, since thedischarge operation continues, resulting in accelerating thedeterioration of the battery. In addition, if an excessive voltage isoutput from the discharger 10A and exceeds the allowable voltage rangeof the load 40 due to a failure or the like in the discharger 10A, loadequipment fails. Furthermore, even if there is a need to stop dischargefrom the discharger 10A due to some reason, there is no means forexternally stopping discharge operation to the load 40 side.

As a method of solving the above problems in the discharger 10A in FIG.4, a method of adding a disconnection unit which stops dischargeoperation to the discharger 10A is available. FIG. 5 is a circuitdiagram showing an example of a circuit arrangement in which adisconnection unit is added to the step-up/step-down DC-DC convertermounted in a discharger, and shows a case in which a disconnection unit11 c for stopping discharge operation is added to the step-up unit 11 aand the step-down unit 11 b.

Although the circuit arrangement of a step-up unit 11 a and step-downunit 11 b in a discharger 10B in FIG. 5 is totally the same as that inthe discharger 10A in FIG. 4 except that a disconnection unit 11 c isfurther connected to the step-up unit 11 a side. The disconnection unit11 c is constituted by a diode 2 c and a disconnection switch 4 c.Controlling the disconnection switch 4 c of the disconnection unit 11 ccan continue or stop discharge operation to the load 40 side of thedischarger 10B.

In the case of the discharger 10B in FIG. 5, the following problemremains unsolved. The function of the disconnection unit 11 c needs toinclude a recovery function which restores discharge operation to theload 40 side when restoration conditions for restoration from adisconnected state hold, in addition to all the functions like thosedescribed above, including an overdischarge prevention function, anovervoltage prevention function, and a function for disconnection fromoutside in accordance with an external signal.

In addition, the discharger 10B generally has the followingcharacteristic. Assume that the discharger detects the occurrence ofoverdischarge by using a threshold set in advance as a voltage value forthe prevention of overdischarge and causes the disconnection unit 11 cto operate to perform disconnection so as to stop power supply to theload 40. In this case, after the disconnection, the battery voltage ofthe assembled battery 30 is naturally restored. For this reason, if theabove threshold is used as a condition for restoration fromdisconnection, and whether to make the disconnection unit 11 c operateor to perform restoration is determined on the basis of only thethreshold, the discharger repeatedly performs disconnection andrestoration, resulting in a state in which the function for protectingthe battery against overdischarge fails to work.

The following problem also arises in the arrangement of the discharger10B. The disconnection unit 11 c is series-connected to the step-up unit11 a and the step-down unit 11 b in the form of additional insertion,and the respective circuits include circuit elements which cause voltagedrops, such as the diodes 2 a and 2 c and the disconnection switch 4 c.For this reason, even if the discharger 10B is made to operate in thebypass mode, a voltage difference occurs between the input and output ofthe discharger 10B, and the output performance of the battery systemdeteriorates.

In the arrangement of the discharger 10B, voltage drops caused by thediodes 2, i.e., 2 a, 2 b, and 2 c, in the step-up unit 11 a, thestep-down unit 11 b, and disconnection unit 11 c increase the powerloss, and also increase the amount of heat generated by the discharger10B. As a result, part of the dischargeable energy stored in theassembled batteries 30 becomes a loss. This shortens the time duringwhich power can be supplied from the battery system to the load 40, andmakes it necessary to install more assembled batteries 30 (storagebatteries). Furthermore, with an increase in the amount of heatgenerated, it is necessary to increase the size of the discharger 10Band additionally install air conditioning equipment. This also increasesthe installation space for a battery system forming a power supplysystem and the cost necessary for construction.

The disconnection unit 11 c needs to consume power to maintain thecircuit disconnected state of the discharger 10B after disconnectingoperation. Assume that a battery system is constituted by a plurality ofdischargers 10B. In this case, even after all the dischargers 10Bmounted in the battery system are disconnected, it is necessary tomaintain the disconnected state of each discharger 10B. For this reason,the dischargers 10B, which are set at zero voltage in the disconnectedstate, cannot supply power from the output side to the correspondingdisconnection units 11 c. It is necessary to supply power from theassembled batteries 30 on the input side of the dischargers 10B.Therefore, the assembled battery 30 always continues power supplyoperation for the disconnection unit 11 c in any state regardless ofwhether power is being supplied from a commercial power supply. Thisincreases the speed of reduction in the capacity of the assembledbattery 30 and the charge/discharge cycle, resulting in a decrease inthe service life of the assembled battery 30.

Note that the above problems in the discharger 10A in FIG. 4 and thedischarger 10B in FIG. 5 are not limited to a battery system whichincludes a plurality of assembled batteries each constituted by acombination of a plurality of nickel metal hydride batteries andsupplies power output from the assembled batteries to a load viacorresponding step-up/step-down DC-DC converters, respectively. That is,these problems include an early deterioration in the assembled batteries30 due to the continuation of discharge, erroneous output of anovervoltage to the load 40, the problem that no external disconnectionmeans for the discharger 10A is available, the problem concerningoperation conditions for disconnection by the disconnection unit 11 cand conditions for restoration from disconnection, the problemconcerning increases in installation space and the cost required for theconstruction of a battery system with an increase in voltage drop due tothe insertion of the diodes 2, i.e., 2 a and 2 c, and the disconnectionswitch 4 c, and the problem that supplying power from the assembledbattery 30 to the disconnection unit 11 c will shorten the service lifeof the assembled battery 30.

For example, such problems also rise in a secondary battery system whichhas a plurality of assembled batteries each constituted by a combinationof secondary batteries other than nickel metal hydride batteries, e.g.,lithium ion batteries and supply power output from the assembledbatteries to a load via corresponding step-up/step-down DC-DCconverters, a battery system which supplies power output from aplurality of assembled batteries each constituted by a plurality ofbatteries including primary batteries to a load via correspondingstep-up/step-down DC-DC converters, and a power supply system whichsupplies power output from a plurality of power supplies eachconstituted by a plurality of DC power supplies including power storagecapacitors to a load via corresponding step-up/step-down DC-DCconverters.

The present invention has been made in consideration of the aboveproblems, i.e., an early deterioration in assembled batteries due to thecontinuation of discharge, erroneous output of an overvoltage to a load,the problem that no external discharge disconnection means is available,the problem concerning operation conditions for a disconnection unit fordisconnecting a discharger from a load and conditions for restorationfrom disconnection, the problem concerning increases in installationspace and the cost required for the construction of a battery systemwith an increase in voltage drop due to the insertion of diodes fordisconnection, and the problem that always supplying power from anassembled battery to the disconnection unit will shorten the servicelife of the assembled battery.

That is, it is an object of the present invention to provide adischarger and discharge control method which can prevent overdischargeof a battery as a DC power supply, protect load equipment against anovervoltage, perform external disconnection, properly executedisconnection of discharge operation from a load and restoration fromdisconnection in accordance with predetermined conditions, reduce avoltage drop in the discharger, and prolong a battery life byeliminating the necessity to always supply power from the battery to thedischarger.

Means of Solution to the Problem

A discharger of the present invention comprises step-down meansincluding at least a switching element provided between a DC powersupply and an output to a load, and control means for controlling anoutput voltage to the load by controlling the switching element, and ifa predetermined condition holds, stopping power output to the load bysetting the switching element in an open state.

A discharge control method of the present invention comprises thedischarging step of controlling an output voltage to a load bycontrolling a switching element in step-down means including at leastthe switching element provided between a DC power supply and an outputto the load, and the opening step of stopping power output to the loadby setting the switching element in an open state when a predeterminedcondition holds.

Effects of the Invention

As described above, according to the present invention, dischargeoperation to the load can be stopped by providing the discharger withthe control means for setting the switching element in an open statewhen a predetermined condition holds. In the present invention, sincethe switching element of the step-down means is used not only as aconstituent element of a step-down DC-DC converter but also as anelement which implements a discharge disconnection function for stoppingdischarge to the load, there is no need to newly provide a dedicateddisconnection unit as a discharger as in the prior art, and a power losscan be reduced. This can reduce the amount of heat generated. As aconsequence, the present invention can efficiently discharge powerstored in the battery to the load and suppress the space and costrequired to construct a battery system which forms a power supplysystem.

The present invention can also stop discharge operation to a load byexternally issuing an instruction to stop power output to a load, asneeded. If, for example, an overdischarge state of the battery or afailure in the discharger is detected, power supply to the load can bestopped by inputting, to the discharger, an external signal whichinstructs to stop discharge operation. This can prevent overdischarge ofthe battery and prevent the danger of applying an overvoltage to theload due to a failure or the like in the discharger.

According to the present invention, when the discharge operation stopstate is released, the discharge operation of the discharger can beeasily resumed by resetting the external signal input from outside.

The present invention can stop discharge operation to a load whendetecting that an input voltage from a DC power supply becomes equal toor less than a predetermined first set value. As a consequence, thepresent invention can automatically prevent the overdischarge of abattery.

According to the present invention, when it is detected that an inputvoltage from a DC power supply has become equal to or more than a secondset value larger than the first set value after the input voltage fromthe DC power supply has become equal to or less than the first set valueand the switching element has been set in an open state, restoration todischarge operation can be properly performed by setting the switchingelement in either a switching operation state or a short-circuitedstate.

The present invention can stop discharge operation to a load whendetecting that an output voltage to the load has become equal to or morethan the first set value. As a consequence, the present invention canautomatically stop discharge operation to a load even when an abnormallyhigh voltage appears in an output from the discharger due to a failureor the like in the discharger. This can prevent a situation in which anovervoltage is applied to the load and contribute to the protection ofload equipment.

In addition, according to the present invention, since power to thecontrol means is supplied from the output side of the discharger insteadof the battery in a normal state, unnecessary consumption of the batterycan be suppressed. This can reduce the charge/discharge frequency of thebattery and prolong the service life of the battery.

Assume that in a battery system forming a power supply system accordingto the present invention, all the plurality of dischargers connected inparallel with the load are in the discharge stop state. Even in thiscase, replacing one or more of batteries input to the respectivedischargers and operating a manual switch provided for the correspondingdischarger can resume power supply from the new battery, i.e., the inputside of the discharger, to the control means of the discharger. Thismakes it possible to resume discharge operation to the load. Afterdischarge operation is resumed, it is possible to supply power from theoutput sides of the dischargers to all the control means of theplurality of dischargers. Therefore, discharge operation to the load ofthe battery system can be easily started or resumed.

Furthermore, the present invention uses the switching element of thestep-down means as an element which maintains the open state during aperiod in which no power is supplied to the control means. This canmaintain the discharge stop state for the load when no power is suppliedto the control means. This can therefore reliably prevent a situation inwhich the discharger becomes out of control and a battery voltage higherthan the allowable voltage range of the load is output to the load side.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a circuit diagram for explaining an example of the arrangementof a discharger according to an embodiment of the present invention;

FIG. 2 is a flowchart for explaining an example of the control operationof a control unit in the discharger according to the embodiment of thepresent invention;

FIG. 3 is a view showing the arrangement of a battery system formed byusing a plurality of assembled batteries and a plurality of dischargers;

FIG. 4 is a circuit diagram showing an example of the circuitarrangement of a step-up/step-down DC-DC converter mounted in aconventional discharger; and

FIG. 5 is a circuit diagram showing an example of a circuit arrangementobtained by adding a disconnection unit to the step-up/step-down DC-DCconverter mounted in the conventional discharger.

BEST MODE FOR CARRYING OUT THE INVENTION

An example of the best embodiment of a discharger and discharge controlmethod according to the present invention will be described in detailbelow with reference to the accompanying drawings. The followingdescription will exemplify a power supply system formed by a nickelmetal hydride battery system (battery system) including a DC powersupply constituted by a combination of a plurality of nickel metalhydride batteries. However, the present invention is not limited tothis. For example, the present invention can be applied to a secondarybattery system constituted by a combination of secondary batteries otherthan nickel metal hydride batteries, such as a plurality of lithium ionbatteries, a battery system constituted by a combination of a pluralityof batteries including primary batteries, or a power supply systemconstituted by a combination of a plurality of DC voltage sourcesincluding power storage capacitors.

Overview of Embodiment

An overview of this embodiment will be described first. This embodimentrelates to the discharge operation of a discharger of a power supplysystem, and can be effectively applied to a large-capacity, long-lifebattery system using a nickel metal hydride battery system or the like,in particular. This embodiment is characterized in that dischargeoperation to a load can be stopped by opening a switching elementmounted in a discharger as a step-down DC-DC converter as well as makingthe switching element perform switching operation at a proper switchingfrequency or short-circuiting the element so as to step down powersupplied to the load.

That is, the discharger according to this embodiment can stop dischargeoperation to the load by setting the switching element in an open statewhen receiving an external signal which instructs to stop discharge ordetecting that the input/output voltage of the discharger has exceeded apredetermined discharge operation condition range. This discharger canalso restore the switching element to a switching operation state or ashort-circuited state when detecting that an external signal is reset orthat the input/output voltage of the discharger has satisfied apredetermined restoration condition for restoration from a disconnectedstate. This makes it possible for this embodiment to obtain the effectof effectively preventing overdischarge of a battery, erroneous outputof an overvoltage to a load, and the like.

More specifically, the discharger according to this embodiment preventsconsumption of a battery and erroneous output of an overvoltage to theload by stopping discharge operation to the load. By having a dischargedisconnection function which allows starting discharge operation to theload, the discharger according to this embodiment can disconnectdischarge operation or restore to discharge operation on the basis of acondition for a battery voltage input to the discharger, a condition foran output voltage output from the discharger, or an external signalwhich can externally instruct to stop or resume discharge operation. Inthis case, this embodiment uses the switching element of the step-downunit of the discharger not only as a constituent element of a step-downDC-DC converter but also as an element which implements the dischargedisconnection function.

When the switching element of the step-down unit of the discharger is tobe also used as an element which implements a discharge disconnectionfunction, the step-up unit of the discharger is placed on the DC powersupply (e.g., a battery constituted by a plurality of assembledbatteries) side, with the step-down unit being placed on the load side,and the step-up unit and the step-down unit are connected in series soas to supply power output from the DC power supply to a load via thestep-up unit and the step-down unit.

In such a circuit arrangement, the control unit of the discharger stopsdischarge operation to the load side by setting the switching element ofthe step-down unit in an open state on the basis of an input/outputvoltage condition for the discharger or an external signal. If acondition for restoration from the open state of the switching elementholds, the control unit performs control to resume discharge operationto the load side by setting the switching element of the step-down unitin a switching operation state or a short-circuited state.

In addition, the discharger is configured to supply power from theoutput side of the discharger to the power supply of the control unit ofthe discharger in normal times and to supply power from the input sideof the discharger, i.e., the DC power supply side, when a user presses amanual switch. If no power for the power supply of the control unit canbe supplied, the switching element of the step-down unit is set in anopen state to stop discharge operation to the load side.

With this arrangement, this embodiment need not supply unnecessary powerfrom the DC power supply to the discharger. In addition, when powersupply from the output side of the discharger to the control unit stops,the embodiment can resume the operation of the discharger by supplyingpower from the DC power supply on the input side of the discharger.

Arrangement of Embodiment

An example of the arrangement of the discharger according to thisembodiment will be described next with reference to FIG. 1. FIG. 1 is acircuit diagram for explaining an example of the arrangement of thedischarger according to this embodiment, and shows an example of acircuit arrangement having a step-up/step-down DC-DC converter function,a discharge operation disconnection function, and a restorationfunction.

The overall arrangement of a battery system in this embodiment, which isformed by using assembled batteries 30, each constituted by a pluralityof nickel metal hydride batteries, a plurality of dischargers 10, and aplurality of chargers 20, is the same as that shown in FIG. 3. As in thecircuit arrangement of the discharger 10A shown in FIG. 4, eachdischarger 10 is configured to control an output voltage value andsupply power to a load 40 via both a step-up unit 11 a, which is astep-up means for outputting an input voltage from the assembled battery30 as a DC power supply upon stepping up the voltage or without changingit, and a step-down unit 11 b, which is a step-down means for outputtingan input voltage from the assembled battery 30 upon stepping down thevoltage or without changing it. The step-up unit 11 a of the discharger10 includes at least a reactor 1 a, a diode 2 a, a capacitor 3 a, and aswitching element 4 a. The step-down unit 11 b includes at least areactor 1 b, a diode 2 b, a capacitor 3 b, and a switching element 4 b.

A control unit 12 as a control means for controlling the overalloperation of the discharger 10 steps up or down an input voltage Vinfrom the assembled battery 30 to a desired voltage by making theswitching elements 4, i.e., 4 a and 4 b, perform switching operation ata proper switching frequency, and outputs the resultant voltage as anoutput voltage Vout to the load 40. Alternatively, the control unit 12outputs the input voltage Vin from the assembled battery 30 as theoutput voltage Vout to the load 40 without any change by opening theswitching element 4 a and short-circuiting the switching element 4 b, orstops discharge operation to the load 40 by opening the switchingelement 4 b.

In order to make the output voltage Vout to the load 40 fall within theoperating voltage range of the load 40, the control unit 12 monitorsoutput voltages from the step-up unit 11 a and step-down unit 11 b andcontrols the switching frequencies of the switching elements 4 a and 4 bso as not to make an output voltage from the step-up unit 11 a fallbelow a predetermined set value V4 or not to make an output voltage fromthe step-down unit 11 b exceed a predetermined set value V5.

The voltage output from the assembled battery 30 is applied to the load40 after being stepped up or down by the step-up unit 11 a or thestep-down unit 11 b, or is applied to the load 40 without any change bysetting the switching element 4 a of the step-up unit 11 a in an openstate and setting the switching element 4 b of the step-down unit 11 bin a short-circuited state. Alternatively, the application of thevoltage to the load 40 is stopped by setting the switching element 4 bof the step-down unit 11 b in an open state.

The control unit 12 always monitors the input voltage Vin (the inputvoltage from the assembled battery 30) to the discharger 10 and theoutput voltage Vout (the output voltage to the load 40) from thedischarger 10. If an input/output voltage condition set in advance forstopping discharge operation holds, the switching element 4 b of thestep-down unit 11 b is set in an open state to stop discharge operationto the load 40 side. If an input/output voltage condition set in advancefor resuming discharge operation holds thereafter, the control unit 12resumes discharge operation to the load 40 side by setting the switchingelement 4 b of the step-down unit 11 b in a switching operation state ora short-circuited state.

The control unit 12 always monitors the input of an external signal froman external controller 70 which instructs to stop discharge operation tothe load 40. Upon receiving the external signal, the control unit 12stops discharge operation to the load 40 side by setting the switchingelement 4 b of the step-down unit 11 b in an open state. When detectingthereafter that the external signal is reset, the control unit 12resumes discharge operation to the load 40 side by setting the switchingelement 4 b of the step-down unit 11 b in a switching operation state ora short-circuited state.

An external signal will be described below. The controller 70 whichcontrols the overall system including the respective constituentelements is generally mounted in the battery system shown in FIG. 3. Thecontroller 70 issues a charge start/end command to the charger 20 and adischarge permission/inhibition command to the discharger 10, detectsthe occurrence of a failure, and returns measurement data in response toa command from a host apparatus (not shown).

In the battery system in FIG. 3, if the discharger 10 detects a decreasein the input voltage Vin and stops discharge by itself before thecontroller 70 issues a command, the controller 70 outputs a warning,assuming that it has detected a failure in the discharger 10. Inpractical operation, therefore, for example, a set value V1 in thedischarger 10 is set to 38 V, and a battery voltage at which thecontroller 70 stops discharge is set to 40 V so as to make thecontroller 70 always stop discharge first. That is, when the inputvoltage Vin becomes equal to or less than 40 V, the controller 70outputs an external signal to the control unit 12. Discharge stop by thedischarger 10 based on the set value V1 is a safeguard against a case inwhich the controller 70 cannot stop discharge.

Note that power for the control unit 12 is supplied from the output sideof the discharger 10, i.e., the node between itself and the load 40, innormal times. When a manual switch 13 is pressed, this power can besupplied from the input side of the discharger 10, i.e., the assembledbattery 30. The manual switch 13 is not limited to a type that ispressed to be closed in accordance with pressing operation, and may beany type, e.g., a type that is rotated.

This embodiment exemplifies the case in which the control unit 12 isimplemented by hardware. However, the present invention is not limitedto this. For example, the control unit 12 may be implemented by acomputer which can execute a program (discharge control program) tocontrol the discharge operation of the discharger 10 by executing theprogram. When the control unit 12 is to be implemented by a computer, itsuffices to record a discharge control program for controlling thedischarge operation of the discharger 10 in a recording medium such as acomputer-readable ROM or a flash memory and make the computer performoperation.

Note that this embodiment exemplifies the case in which the discharger10 includes both the step-up unit 11 a and the step-down unit 11 b as astep-up/step-down DC-DC converter of the discharger 10. However, if itsuffices to form a discharger which provides only a step-down DC-DCconverter function, the discharger may include only the step-down unit11 b.

In addition, the step-up unit 11 a and the step-down unit 11 b canfurther include, for example, voltage stabilization circuits forstabilizing output voltages.

Operation of Embodiment

An example of the control operation of the control unit 12 of thedischarger 10 shown in FIG. 1 will be described next with reference tothe flowchart of FIG. 2. The control method (discharge control method)for the discharge operation of the discharger 10 which is shown in FIG.2 can be implemented by the execution of the discharge control programor by reading the discharge control program from a program recordingmedium as well as by hardware. As a program recording medium whichrecords the discharge control program, a ROM or flash memory which canbe incorporated in the control unit 12 or a ROM or flash memory whichcan be externally attached to the control unit 12 can be used.Alternatively, if the control unit 12 has a reading function for aportable recording medium such as a USB memory, memory card, FDD, CD, orDVD, the program may be recorded on such a portable recording medium.

Immediately after the start of control operation, based on the monitorresult of the output voltage Vout from the step-down unit 11 b, thecontrol unit 12 of the discharger 10 maintains the output voltage Vout(i.e., the power supply voltage applied to the load 40) at a requiredvoltage value equal to or less than a predetermined set value V5 bycausing the switching element 4 b of the step-down unit 11 b to performswitching operation at a proper switching frequency or short-circuitingthe switching element 4 b (step S1 in FIG. 2).

Upon receiving an external signal which instructs to stop dischargeoperation from the controller 70 (YES in step S2), the control unit 12sets the switching element 4 b of the step-down unit 11 b in an openstate so as to stop discharge operation to the load 40 (step S3).

Thereafter, when the control unit 12 detects that an external signalwhich instructs to stop discharge operation has been reset (YES in stepS4), the process returns to step S1 to restore the switching element 4 bof the step-down unit 11 b to the switching operation state or theshort-circuited state so as to resume discharge operation to the load40.

Upon detecting that the input voltage Vin to the discharger 10 hasbecome equal to or less than a set value V1 (e.g., 40 V) set in advancefor the prevention of overdischarge of the assembled battery 30 (YES instep S5) while not receiving the above external signal (NO in step S2),the control unit 12 sets the switching element 4 b of the step-down unit11 b in an open state to stop discharge operation to the load 40 (stepS6).

Subsequently, when the input voltage Vin to the discharger 10 isrestored by replacing the assembled battery 30, and the control unit 12detects that the input voltage Vin has become equal to or more than aset value V2 (e.g., 50 V) set in advance as a voltage value larger thanthe set value V1 (YES in step S7), the process returns to step S1 torestore the switching element 4 b of the step-down unit 11 b to theswitching operating state or the short-circuited state so as to resumedischarge operation to the load 40.

The set value V2 (e.g., 50 V) provided as a condition for restoration todischarge operation is set to be larger than the set value V1 (e.g., 40V) as described above considering also the prevention of repetition ofdischarge stop and restoration due to the characteristic that thebattery voltage of the assembled battery 30 naturally restores afterdischarge stop.

The range of the input voltages Vin to the discharger 10 (the batteryvoltages output from the assembled battery 30) is a predeterminedvoltage range, e.g., the voltage range of 40 V to 64 V. For this reason,the discharger 10 makes the output voltage Vout discharged to the load40 fall within a desired operating voltage range in the allowablevoltage range of the load 40 by outputting the input voltage Vin afterstepping up or stepping down it or without changing it. If, however, forexample, the reactor 1 b is short-circuited due to a failure or the likein the discharger 10, the output voltage Vout from the discharger 10sometimes abnormally rises. In some cases, this output voltage maybecome an overvoltage exceeding the allowable voltage range of the load40 and cause a failure in the load equipment.

For this reason, upon detecting that the output voltage Vout from thedischarger 10 has become equal to or more than a set value V3 (e.g., 53V) set in advance for the prevention of output of an overvoltage to theload 40 (YES in step S8), the control unit 12 sets the switching element4 b of the step-down unit 11 b in an open state to stop discharge to theload 40 (step S9). This protects the load equipment againstovervoltages. Thereafter, it is necessary to execute a repair processfor removing the cause of a failure or the like in the discharger 10which has caused an overvoltage.

If the control unit 12 determines that the output voltage Vout from thedischarger 10 is smaller than the set value V3 (e.g., 53 V) (NO in stepS8), since this indicates a state in which the normal output voltageVout is maintained, the process returns to step S2 to sequentiallyrepeat the following operations: checking whether an external signalwhich instructs to stop discharge operation has been received (step S2),checking whether the input voltage Vin to the discharger 10 has becomeequal to or less than the set value V1 (step S5), and checking whetherthe output voltage Vout from the discharger 10 has become equal to ormore than the set value V3 (step S8).

Note that the set value V3 (e.g., 53 V) provided for the prevention ofoutput of an overvoltage is a value set to be equal to or less than theupper limit value of the allowable voltage range allowed by the load 40in consideration of the safety of the load equipment. That is, the setvalue V3 is a value set on the basis of the allowable voltage range ofthe load 40, and is a value independent of the variation range ofbattery voltages output from the assembled battery 30. Depending on thenumber of nickel metal hydride batteries constituting the assembledbattery 30 which are connected in series, power supply voltages fallwithin the variation range of 40 to 64 V. In some applications,therefore, the set value V3 can be a value smaller than the set value V1(e.g., 40 V) indicating a voltage value for the prevention ofoverdischarge of the assembled battery 30, i.e., a value smaller thanthe lower limit value of battery voltages to be output from the normalassembled battery 30.

Assume that the switching element 4 b of the step-down unit 11 b isformed by using an FET (Field-Effect Transistor), and the gate terminalof the FET is connected to the control output terminal of the controlunit 12 to control the gate potential of the FET in accordance with avoltage at the control output terminal. In this case, the FET can beturned on/off depending on the presence/absence of a voltage at thecontrol output terminal, and hence the step-down operation of thestep-down unit 11 b can be controlled. In this case, when no power issupplied to the control unit 12, the gate potential of the FET formingthe switching element 4 b is set to zero, and hence the switchingelement 4 b is automatically set in an open state. For this reason, whenno power is supplied to the control unit 12, the switching element 4 bis automatically opened to forcibly stop discharge operation to the load40. This can prevent an abnormal overvoltage from being output from thedischarger 10 to the load 40 even if the discharger 10 is set in anuncontrollable state due to the interruption of power supply to thecontrol unit 12.

The power supply for the control unit 12 is wired such that power issupplied from the output side of the discharger 10. In the arrangementin which a plurality of dischargers 10 are connected in parallel witheach other to perform discharge operation to the load 40 as shown inFIG. 3, even if the switching element 4 b of the step-down unit 11 b inthe discharger 10 is opened, since power supply from another discharger10 to the control unit 12 of the discharger 10 continues, the controlunit 12 can continue control operation.

If, however, only one discharger 10 is used, when the switching element4 b of the step-down unit 11 b is opened, power supply to the controlunit 12 of the discharger 10 is simultaneously interrupted and disabled.Even in such a case, if an element like an FET as the switching element4 b is connected to the control output terminal of the control unit 12as in the above arrangement, the open state of the switching element 4 bof the step-down unit 11 b can be maintained.

If the switching elements 4 b of the step-down units 11 b in all thedischargers 10 are opened in an application using a battery system likethat shown in FIG. 3, in which the plurality of dischargers 10 areconnected in parallel on the output side, since the output voltage Voutbecomes zero, no power is supplied to the control units 12 of all thedischargers 10. As a consequence, no power is supplied to the controlunits 12 of all the dischargers 10, and the control units 12 of all thedischargers 10 stop control operation. Therefore, the operation of allthe dischargers 10 completely stop operating.

In such a case, in order to activate the control unit 12 of thedischarger 10 again, it is necessary, in, for example, a battery systemlike that shown in FIG. 3, to restore a commercial power supply in afailed state and make the rectifier 50 resume outputting. When thecommercial power supply is restored to supply power from the rectifier50 to the discharger 10, the control unit 12 of the discharger 10 isactivated. This makes it possible to perform control operation for thedischarger 10.

Assume that the operation of the control unit 12 of the discharger 10 isto be activated again by replacing the assembled battery 30 instead ofrestoring the commercial power supply which has been in the failedstate. Pressing the manual switch 13 in FIG. 1 can make the discharger10 resume discharge to the load 40 of the discharger 10. That is, whilethe manual switch 13 is pressed, a power supply line from the assembledbattery 30 on the input side of the discharger 10 is formed for thecontrol unit 12 as well as on the output side of the discharger 10. Forthis reason, the control unit 12 is activated by power from theassembled battery 30 which has replaced the old assembled battery torestore the switching element 4 b of the step-down unit 11 b to theswitching operation state or the short-circuited state. This resumesdischarge to the load 40.

In a battery system in which a plurality of dischargers 10 are connectedin parallel on the output side as shown in FIG. 3, when a givendischarger 10 resumes discharge operation to the load 40, power supplyto the control units 12 of the other dischargers 10 connected inparallel is started via the output line of the discharger 10. If,therefore, the effective assembled battery 30 capable of outputting anormal battery voltage is connected to any one of the dischargers 10,since discharge to the load 40 is resumed, it suffices to press only themanual switch 13 of one discharger 10, of the plurality dischargers 10connected in parallel, to which the effective assembled battery 30 isconnected. The control unit 12 is based on the assumption that it canoperate at all voltage values within the battery voltage variation rangeof the assembled battery 30 and the output voltage variation range ofthe discharger 10.

As described in detail above, a characteristic feature of the discharger10 according to this embodiment is that the switching element 4 b of thestep-down unit 11 b is used not only as a constituent element of thestep-down DC-DC converter but also as an element which implements adisconnection function for discharge operation to the load 40.

For example, upon receiving an external signal which externallyinstructs to stop discharge operation, the control unit 12 of thedischarger 10 opens the switching element 4 b of the step-down unit 11 bto stop discharge to the load 40. Thereafter, upon detecting that theexternal signal has been reset, the control unit 12 performs control toresume supply of power to the load 40 by restoring the switching element4 b of the step-down unit 11 b to the switching operation state or theshort-circuited state.

Furthermore, upon detecting that the input voltage Vin to the discharger10 has become equal to or less than the set value V1 set in advance forthe prevention of overdischarge of the assembled battery 30, the controlunit 12 opens the switching element 4 b of the step-down unit 11 b tostop discharge to the load 40. Thereafter, upon detecting that the inputvoltage Vin to the discharger 10 has become equal to or more than theset value V2 set in advance as a value larger than the set value V1 dueto replacement of the assembled battery 30 or the like, the control unit12 performs control to resume supply of power to the load 40 byrestoring the switching element 4 b of the step-down unit 11 b to theswitching operation state or the short-circuited state.

Moreover, upon detecting that the output voltage Vout from thedischarger 10 has become equal to or more than the set value V3 set inadvance for the prevention of output of an overvoltage to the load 40,the control unit 12 performs control to stop discharge to the load 40 byopening the switching element 4 b of the step-down unit 11 b, in orderto prevent an overvoltage from being applied to the load equipment.

In normal times during which the manual switch 13 is not pressed, powerfor the power supply for the control unit 12 is supplied from the outputside of the discharger 10. In contrast to this, while the manual switch13 is pressed, power can also be supplied from the input side of thedischarger 10. In a state in which no power for the power supply for thecontrol unit 12 can be supplied, the switching element 4 b of thestep-down unit 11 b is maintained in the opened state.

Note that the above embodiment has exemplified the case in which thebatteries constituting the assembled battery 30 are nickel metal hydridebatteries. However, the present invention is not limited to this, asdescribed above.

By having the above characteristics, this embodiment can preventoverdischarge of the assembled battery 30 by stopping dischargeoperation in accordance with the input/output voltage of the discharger10. In addition, the embodiment can protect the load 40 against anovervoltage and can stop discharge in accordance with external controloperation. In addition, the embodiment properly performs restorationfrom a discharge stop state (the disconnected state of the load 40).

This embodiment uses the switching element 4 b of the step-down unit 11b not only as a constituent element of the step-down DC-DC converter butalso as an element which implements a disconnection function fordischarge operation to the load 40, and hence eliminates the necessityto additionally insert a new disconnection unit dedicated to adisconnection function in the power supply system. This embodiment istherefore free from increases in power loss and the amount of heatgenerated with increases in voltage drop as in a case in which a newdisconnection unit is additionally inserted, and hence will not increasethe installation space and the cost required to construct a batterysystem. In addition, it is not necessary to always supply power from theassembled battery 30 to the disconnection unit. This can also solve theconventional problem that the service life of the battery decreases dueto continuous supply of power to the discharger 10.

In this embodiment, in a battery system in which a plurality ofdischargers 10 are connected in parallel on the output side, even afterdischarge from all the dischargers 10 is stopped, resuming supply ofpower from a restored commercial power supply via a rectifier orreplacing one of the plurality of assembled batteries 30 can start thebattery system by operating the manual switch 13.

Effects of Embodiment of Present Invention

The effects obtained by the embodiment of the present invention will bedescribed next in further detail.

First, in order to prevent overdischarge of a battery, protect loadequipment against an overvoltage, and allow to externally stop dischargeoperation to the load, it is necessary for the discharger to include ameans for properly stopping and restoring discharge operation to theload.

This embodiment is configured to also use the switching element 4 b ofthe step-down unit 11 b as an element which implements a disconnectionfunction for discharge operation, and hence can prevent overdischarge ofthe assembled battery 30 by setting the switching element 4 b in an openstate under the control of the control unit 12 which is monitoring theinput voltage Vin to the discharger 10. The embodiment can also protectload equipment against erroneous output of an overvoltage by setting theswitching element 4 b in an open state under the control of the controlunit 12 which is monitoring the output voltage Vout from the discharger10. In addition, the embodiment can externally control discharge stop bysetting the switching element 4 b in an open state under the control ofthe control unit 12 which is monitoring the reception of an externalsignal which externally instructs to stop discharge operation.Furthermore, the embodiment can properly resume discharge operation tothe load 40, when a condition for restoration from the disconnectedstate of the load 40 holds, by restoring the switching element 4 b tothe switching operation state or the short-circuited state under thecontrol of the control unit 12 which is monitoring such a restorationcondition.

Second, in a discharger including a disconnection unit for disconnectinga load separately from a step-up/step-down DC-DC converter unit, avoltage drop occurs due to the additional insertion of a circuit elementsuch as a diode forming the disconnection unit in a power supply line,resulting in an increase in power loss in the discharger and anaccompanying increase in the amount of heat generated. It is thereforenecessary to increase the size of the discharger and additionallyinstall air conditioning equipment with an increase in the amount ofheat generated. It is also necessary to increase the space and costrequired to construct a battery system because of the necessity toinstall additional batteries to compensate for power loss in thedischarger.

This embodiment is configured to also use the switching element 4 b ofthe step-down unit 11 b as an element which implements a disconnectionfunction for discharge operation, and hence need not add any newseparation unit and can suppress increases in power loss in thedischarger 10 and the amount of heat generated. Along with this effect,the embodiment allows easy consideration of heat dissipation measuresfor the discharger 10 and can reduce the size of the discharger 10. Inaddition, suppressing an increase in the amount of heat generated caneliminate the necessity to additionally install air conditioningequipment. Furthermore, in the embodiment, since power output from theassembled battery 30 is efficiently supplied to the load 40, there is noneed to add extra batteries. As a consequence, the embodiment can save aspace and cost required to construct a battery system (power supplysystem).

Third, a discharger designed to supply operating power from a battery toa disconnection unit which disconnects a load has a mechanism of alwayssupplying power from the battery to the disconnection unit, and hencethe consumption of the battery is accelerated to increase the chargefrequency of the battery and increase the charge/discharge cycle of thebattery, resulting in the acceleration of deterioration of the battery.

This embodiment is configured to also use the switching element 4 b ofthe step-down unit 11 b as an element which implements a disconnectionfunction for discharge operation, and hence operating power for thestep-down unit 11 b including the switching element 4 b is supplied fromthe output side of the discharger 10 (i.e., the side from which outputpower is supplied to the load 40). This suppresses the consumption ofthe battery, and hence reduces the charge frequency of the battery. As aconsequence, the service life of the battery can be prolonged.

Fourth, a discharger generally includes a control unit, which controlsthe switching operation of the switching element of a step-down unit.Assume that power to the control unit is interrupted. In this case, ifthe switching element of the step-down unit is set in a short-circuitedstate, an overvoltage which is not stepped down may be erroneouslyoutput to the load side. This may cause a problem in the load equipment.

In this embodiment, when no power is supplied to the control unit 12,since the switching element 4 b of the step-down unit 11 b is maintainedin an open state, discharge to the load 40 is stopped. This makes itpossible to maintain the load equipment in a safety state.

Fifth, in a case in which power is supplied from the output side of thedischarger to the control unit of the discharger, when discharge isstopped, since power supply to the control unit is also interrupted, thedischarger itself is left in an inoperable state, regardless of theeffort to restore the discharge operation of the discharger, untildischarge operation to the output side of the discharger is restored.This makes it absolutely necessary to supply power from, for example, acommercial power supply restored from a power failure state to a chargerand a discharger via a rectifier.

This embodiment includes an electric circuit which supplies power fromthe assembled battery 30 on the input side of the discharger 10 to thecontrol unit 12 as well as from the output side of the discharger 10upon operation of the manual switch 13, and can activate dischargeoperation using power from the assembled battery 30. This makes itpossible to activate the operation of the discharger 10 by operating themanual switch 13 after replacement of the assembled battery 30 and toprovide a resumption means independently of the operation of resumingpower supply from a commercial power supply.

INDUSTRIAL APPLICABILITY

The present invention can be applied to a discharger which controls anoutput from a DC power supply which is supplied to a load.

1. A discharger comprising: step-down means including at least aswitching element provided between a DC power supply and an output to aload; control means for controlling an output voltage to the load bycontrolling the switching element, and if a predetermined conditionholds, stopping power output to the load by setting the switchingelement in an open state, wherein said control means includes means fori) determining that the predetermined condition holds, when detectingthat an input voltage from the DC power supply has become not more thana predetermined first set value, and ii) resuming power output to theload by setting the switching element in one of a state of a switchingelement being activated into its switching operation to drop a voltageinput thereto from a DC power supply to a desired voltage to be outputto a load and a short-circuited state when detecting an input voltagefrom the DC power supply has become not less than a second set valuelarger than the first set value after the input voltage from the DCpower supply has become not more than the first set value and theswitching element has been set in an open state.
 2. A dischargeraccording to claim 1, wherein said control means includes means fordetermining that the predetermined condition holds, when receiving, fromoutside, an external signal which instructs to stop power output to theload.
 3. A discharger according to claim 2, wherein said control meansincludes means for resuming power output to the load by setting theswitching element in one of a state of a switching element beingactivated into its switching operation to drop a voltage input theretofrom a DC power supply to a desired voltage to be output to a load and ashort-circuited state, when detecting the external signal is reset afterthe switching element is set in an open state in accordance with theexternal signal.
 4. A discharger comprising: step-down means includingat least a switching element provided between a DC power supply and anoutput to a load; and control means for controlling an output voltage tothe load by controlling the switching element, and if a predeterminedcondition holds, stopping power output to the load by setting theswitching element in an open state, wherein said control means includesmeans for determining that the predetermined condition holds, whendetecting that an output voltage to the load has become not less than apredetermined first set value and means for determining that thepredetermined condition holds, when receiving, from outside, an externalsignal which instructs to stop power output to the load; and whereinsaid control means includes means for resuming power output to the loadby setting the switching element in one of a state of a switchingelement being activated into its switching operation to drop a voltageinput thereto from a DC power supply to a desired voltage to be outputto a load and a short-circuited state, when detecting the externalsignal is reset after the switching element is set in an open state inaccordance with the external signal.
 5. A discharge control methodcomprising: a discharging step controlling an output voltage to a loadby controlling a switching element in step-down means including at leastthe switching element provided between a DC power supply and an outputto the load; an opening step stopping power output to the load bysetting the switching element in an open state when a predeterminedcondition holds, wherein the opening step includes determining that thepredetermined condition holds, when detecting that an input voltage fromthe DC power supply has become not more than a predetermined first setvalue, and resuming power output to the load by setting the switchingelement in one of a state of a switching element being activated intoits switching operation to drop a voltage input thereto from a DC powersupply to a desired voltage to be output to a load and a short-circuitedstate when detecting an input voltage from the DC power supply hasbecome not less than a second set value larger than the first set valueafter the input voltage from the DC power supply has become not morethan the first set value and the switching element has been set in anopen state.
 6. A discharge control method according to claim 5, whereinthe opening step includes determining that the predetermined conditionholds, when receiving, from outside, an external signal which instructsto stop power output to the load.
 7. A discharge control methodaccording to claim 6, further comprising resuming power output to theload by setting the switching element in one of a state of a switchingelement being activated into its switching operation to drop a voltageinput thereto from a DC power supply to a desired voltage to be outputto a load and a short-circuited state, when detecting the externalsignal is reset after the switching element is set in an open state inaccordance with the external signal.
 8. A discharge control methodcomprising: a discharging step of controlling an output voltage to aload by controlling a switching element in step-down means including atleast the switching element provided between a DC power supply and anoutput to the load; and a opening step of stopping power output to theload by setting the switching element in an open state when apredetermined condition holds, wherein the opening step includes thesteps of: determining that the predetermined condition holds, whendetecting that an output voltage to the load has become not less than apredetermined first set value, and determining that the predeterminedcondition holds, when receiving from outside, an external signal whichinstructs to stop power output to the load; and further comprisingresuming power output to the load by setting the switching element inone of a state of a switching element being activated into its switchingoperation to drop a voltage input thereto from a DC power supply to adesired voltage to be output to a load and a short-circuited state, whendetecting the external signal is reset after the switching element isset in an open state in accordance with the external signal.